Apparatus and methods for optical amplification in semiconductors

ABSTRACT

Methods and corresponding apparatus for optical amplification in semiconductors, particularly indirect band-gap semiconductors, and most particularly in silicon. A first aspect of the invention employs certain doping elements to provide inter-band-gap energy levels in combination with optical or current-injection pumping. The doping element, preferably a noble metal and most preferably Gold, is chosen to provide an energy level which enables an energy transition corresponding to a photon of wavelength equal to the signal wavelength to be amplified. The energy transition may be finely “adjusted” by use of standard doping techniques (such as n-type or p-type doping) to alter the conduction and valence band energy levels and thereby also the magnitude of the energy transition. A second aspect of the invention relates to the use of a non-homogeneous heat distribution which has been found to lead to optical amplification effects.

This application is a continuation of U.S. Ser. No. 10/577,858 filed onApr. 28, 2006, which is a National Phase Application ofPCT/IL2004/001027 filed on Nov. 10, 2004, and also claims the benefitunder 119(e) of Provisional Patent Application No. 60/518,341 filed Nov.10, 2003, and Provisional Patent Application No. 60/606,468 filed Sep.2, 2004 the contents of which are incorporated herein by reference.

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to optical amplification and, inparticular, it concerns optical amplification in semiconductors,particularly silicon, by optical pumping and/or non-homogeneous heating.

There is presently a great deal of interest in the pursuing ofsemiconductors as basic materials for lasers and optical amplifiers foroptical networks. Most of existing semiconductor lasers and amplifiersare based on direct band gap semiconductor materials. At present,Silicon is not considered a favorable candidate for these applications,by virtue of indirect band gap structure. On the other hand, lightstimulated emission properties of GaP, also an indirect band-gapsemiconductor has been reported and light-emitting diodes (LEDs) basedon this material are commercially available [1]. Regarding dopedSilicon, a massive effort in order to develop lasers and amplifiers istaking place these days worldwide (see Refs. [2,3] for a review on thesubject). Experimental observation of the stimulated emission in bulkSilicon was not reported to-date to the best of our knowledge [4], sinceBasov, et al [5], and recently Trupke, et al [6] theoretically predictedthis possibility. Reports on gain included Er⁺-doped silicon and silicondioxide structures [7] and structures not based on the originalcrystalline structure (nano-crystals, nano-layers etc) [2]. Gain wasalso attained via non-linear processes e.g. stimulated Raman scattering[8] and multiwave mixing [9], the mechanisms present in many materials.Our main goal here is the disclosure of methods for attaining stimulatedemission in Silicon. Stimulated emission is in the essence of laseraction or optical amplification of light signals.

SUMMARY OF THE INVENTION

The present invention provides two independent methods that can beapplied for developing of silicon based optical amplifiers and lasers.First is based on the photons stimulated emission that is the result offree carriers recombination via impurities (recombination centers,traps) in forbidden band gap. The second method of generation of thephoton's stimulated emission is based on the non-homogeneous heating ofdirect gap or indirect band gap semiconductor materials. Optionally, thetwo methods may be employed simultaneously to advantage.

According to the teachings of the present invention there is provided, amethod for achieving optical amplification of an optical signal passingthrough a semiconductor, the method comprising the steps of: (a)providing a semiconductor material, the semiconductor material having agiven band gap energy at a given temperature; (b) heating thesemiconductor material so as to raise at least a portion of thesemiconductor material to a temperature such that the band gap energy inthe portion is smaller by at least 5% than the band gap at the giventemperature, the heating being performed so as to generate aninhomogeneous temperature distribution within a target volume of thesemiconductor; and (c) directing the optical signal through the targetvolume.

There is also provided according to the teachings of the presentinvention, an apparatus for achieving optical amplification of anoptical signal, the apparatus comprising: (a) a body of semiconductormaterial including a target volume, the semiconductor material having agiven band gap energy at room temperature; (b) a heating arrangementoperatively associated with the body of semiconductor material forraising at least a portion of the semiconductor material to atemperature such that the band gap energy in the portion is smaller byat least 5% than the band gap at the given temperature, the heatingbeing performed so as to generate an inhomogeneous temperaturedistribution within a target volume of the semiconductor; and (c) anoptical arrangement for directing an optical signal through the targetvolume.

According to a further feature of the present invention, thesemiconductor material is an indirect band-gap semiconductor material,and most preferably silicon.

According to a further feature of the present invention, the heating isperformed so as to raise at least a portion of the semiconductormaterial to a temperature in the range of between 200° C. and 1000° C.above an ambient temperature.

According to a further feature of the present invention, the heating isperformed so as to raise at least a portion of the semiconductormaterial to a temperature such that the band gap energy in the portionis smaller by at least 10% than the given band gap energy.

According to a further feature of the present invention, the heating isperformed by directing laser radiation onto a region of thesemiconductor material.

According to a further feature of the present invention, the laserradiation is directed onto a region of the semiconductor material coatedwith a compound having lower reflectivity than an exposed surface of thesemiconductor material.

According to a further feature of the present invention, the opticalsignal is directed into a region of the semiconductor material coatedwith a compound having higher reflectivity than an exposed surface ofthe semiconductor material so as to cause reflection of the opticalsignal so as to pass through the target volume a plurality of times.

According to a further feature of the present invention, the heating isperformed by directing a source of microwave radiation into a region ofthe semiconductor material.

According to a further feature of the present invention, the heating isperformed by directing heat from a non-coherent light source onto aregion of the semiconductor material.

According to a further feature of the present invention, the heating isperformed by passing an electric current through a resistive loadassociated with the semiconductor material.

According to a further feature of the present invention, at least thetarget volume of the semiconductor forms part of an optical waveguide,the step of directing the optical signal being performed by directingthe optical signal along the optical waveguide.

According to a further feature of the present invention, thesemiconductor material is silicon doped with at least one element chosenfrom the group comprising: Gold, Silver, Platinum, Iron, Copper, Zinc,Cobalt, Tellurium, Mercury, Nickel, Sulfur and Manganese.

There is also provided according to the teachings of the presentinvention, a method for achieving optical amplification of an opticalsignal passing through indirect-gap semiconductor, the method comprisingthe steps of: (a) providing a body of the indirect-gap semiconductordoped with at least one element so as to generate at least one addedenergy level at a known energy lying within the energy band-gap of thesemiconductor, the added energy level enabling an energy transitionbetween the added energy level and an energy band of the semiconductorcorresponding to generation of a photon of a given wavelength; (b)irradiating a target region of the body of semiconductor with opticalillumination of a wavelength shorter than the given wavelength; and (c)directing an optical signal of the given wavelength through the targetregion.

According to a further feature of the present invention, theillumination has a wavelength no greater than a wavelength of a photoncorresponding to the transition between the conduction gap and thevalence band in the semiconductor.

According to a further feature of the present invention, the at leastone element is chosen from the group comprising: Gold, Silver, Platinum,Iron, Copper, Zinc, Cobalt, Tellurium, Mercury, Nickel, Sulfur andManganese, More preferably, the at least one element is chosen from thegroup comprising: Gold, Silver and Platinum. Most preferably, the atleast one element includes Gold.

According to a further feature of the present invention, the givenwavelength is in the range of 1.2-2.2 micrometers.

According to a further feature of the present invention, the irradiatingis performed using a pulsed laser source.

According to a further feature of the present invention, the irradiatingis performed using a substantially continuously irradiating lasersource.

According to a further feature of the present invention, the targetregion lies at least partially in an optical waveguide formed in thebody of semiconductor.

According to a further feature of the present invention, theindirect-gap semiconductor is silicon.

There is also provided according to the teachings of the presentinvention, a method for achieving optical amplification of an opticalsignal passing through an indirect-gap semiconductor, the methodcomprising the steps of: (a) providing a body of the indirect-gapsemiconductor doped with at least one element so as to generate at leastone added energy level at a known energy lying within the energyband-gap of the semiconductor, the added energy level enabling an energytransition between the added energy level and an energy band of thesemiconductor corresponding to generation of a photon of a givenwavelength; (b) performing current injection into at least a targetregion of the body of semiconductor; and (b) directing an optical signalof the given wavelength through the target region.

According to a further feature of the present invention, the at leastone element is chosen from the group comprising. Gold, Silver, Platinum,Iron, Copper, Zinc, Cobalt, Tellurium, Mercury, Nickel, Sulfur andManganese.

According to a further feature of the present invention, theindirect-gap semiconductor is silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1 is a schematic illustration of a hypothetical mechanism of lightamplification in Gold-doped Silicon;

FIG. 2 is an experimental set-up used for measurements of lightamplification;

FIG. 3 shows oscillograms from photodetector measurements obtained fromthe apparatus of FIG. 2, wherein the dotted line shows infrared signalresponse on pump influence and the straight line shows measurements whenreference signal was shutdown and at pumping action.

FIG. 4 is a graph showing the dependence of the infrared signaltransmission through optically pumped SOI-based waveguides on the pumppower, wherein circles indicate measurements taken with a gold-dopedwaveguide and squares indicate measurements taken in a sample withoutgold doping;

FIG. 5A illustrates schematically an apparatus according to a secondaspect of the present invention for delivering heating power for asample irradiated at the edge by a focused laser beam;

FIG. 5B is similar to FIG. 5A, but illustrates an on-plane laser heatingpower delivery scheme;

FIG. 6 shows signal transmissions for different pump powers anddifferent lateral spatial positions of signal relative to pump spotwherein:

-   -   Oscillogram (a) shows transmission of a 1.5 μm-wavelength signal        through a 0.37-mm thick silicon sample with antireflective        coating that was edge pumped at 15 mJ-power in single pulse;        lines are: (1) signal with pumping, (2) signal with silicon slab        removed, and (3) pump laser trace; and    -   Oscillograms (b)-(e) show low coherence (spontaneous light        emission from an EDF) 1.5 μm-wavelength signal transmissions        though 1-mm thick silicon sample that was pumped at the same        side and for different space positions, and pump power, namely:        (b), and (c)-original position, pump power was 40 mJ/pulse, and        160 mJ/pulse respectively; (d), and (e)-pump power was 40        mJ/pulse, and moving of the signal beam position were 0.8 and        1.28 mm, respectively;

FIG. 7 shows dependence of the optical gain on pump power for differentsignal sources and different silicon samples wherein:

-   -   Curves of figure (A) show light amplification for 0.37-mm-thick        n-type (˜5 Ωcm) phosphorus-doped Silicon sample with SiO        antireflective coating: circles are for EDF spontaneous light        emission, triangles for 1.54-μm laser, and squares for 1.3-μm        laser sources;    -   Curves of figure (B) show light amplification for EDF source        used as signal: hollow circles are for 1-mm-thick        phosphorus-doped (˜50 Ωcm), hollow triangles for 0.5-mm-thick        boron-doped (˜50 Ωcm), and hollow squares for 0.37-mm-thick        n-type (˜5 Ωcm) phosphorus-doped silicon samples;

FIG. 8 is a graph showing the maximum gain in quasi-CW irradiatedsamples, as a function of pump/heating pulse duration wherein the dottedline is for continuous laser excitation and the pump power for bothmeasurements was kept constant at 5 Watt;

FIG. 9 shows luminescence spectra for different light pump power;

FIG. 10 shows the dependence of a reference light signal transmission onposition of a pump spot relative to a waveguide;

FIG. 11 shows the dependence of an amplification and attenuation of theinfrared light signal on pump pulse time in SOI-based waveguides,wherein solid circles show signal amplification when pump spot was movedfrom waveguide center to a few tens microns and hollow circles showsignal attenuation when pump spot was centered on waveguide;

FIGS. 12A and 12B show two preferred schemes for non-homogeneous heatingof silicon samples by a resistive load applied to the silicon; and

FIG. 13 shows graphs illustrating: (A) energy states deformation inlocal heated semiconductor material (shown as discrete states forillustration purposes); and (B) schematic distribution of intrinsic andnon-equilibrium carrier densities following local heating.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to methods and corresponding apparatus forachieving optical amplification in semiconductors, particularly indirectband-gap semiconductors, and most particularly in silicon. A firstaspect of the invention relates to the use of certain doping elements toprovide inter-band-gap energy levels in combination with optical and/orcurrent-injection pumping. The doping element, preferably a noble metaland most preferably Gold, is chosen to provide an energy level whichenables an energy transition corresponding to a photon of wavelengthequal to the signal wavelength to be amplified. The energy transitionmay be finely “adjusted” by use of standard doping techniques (such asn-type or p-type doping) to alter the conduction and valence band energylevels and thereby also the magnitude of the energy transition. A secondaspect of the invention relates to the use of a non-homogeneous heatdistribution which has been found to lead to optical amplificationeffects.

Thus, in general terms, the apparatus for light-pumped amplificationconstructed and operative according to the teachings of the presentinvention as shown in FIG. 2 includes a body of silicon doped with atleast one element so as to generate at least one added energy level at aknown energy lying within the energy band-gap of the silicon, the addedenergy level enabling an energy transition between the added energylevel and an energy band of the silicon corresponding to generation of aphoton of a given wavelength. The apparatus further includes a firstoptical arrangement for irradiating a target region of the body ofsilicon with optical illumination of a wavelength shorter than the givenwavelength, and a second optical arrangement for directing an opticalsignal of the given wavelength through the target region.

The first optical arrangement preferably illuminates with radiationhaving a wavelength no greater than a wavelength of a photoncorresponding to the transition between the conduction gap and thevalence band in the silicon. The first optical arrangement preferablyincludes a pulsed laser source. Alternatively, a substantiallycontinuously irradiating laser source (i.e., either with continuousoutput or high duty-cycle pulses approaching continuous output) may beused.

The at least one element is chosen from the group comprising: Gold,Silver, Platinum, Iron, Copper, Zinc, Cobalt, Tellurium, Mercury,Nickel, Sulfur and Manganese. More preferably, a noble metal (Gold,Silver or Platinum) is used, and most preferably, Gold.

Most preferably, the primary doping element is chosen, optionallytogether with a secondary doping element for modifying the conductionand/or valence band energy levels of the semiconductor, so as to achieveoptical amplification for wavelengths in the near infrared range, apreferred range being between 1.2 and 2.2 micrometers.

The target region within which amplification occurs is most preferablyat least partially within an optical waveguide formed in the body ofsilicon.

Further in general terms, the optical amplification apparatus accordingto the second aspect of the present invention as exemplified in FIGS. 5Aand 5B includes a body of semiconductor material including a targetvolume, the semiconductor material having a given band gap energy atroom temperature. A heating arrangement is deployed to raise at least aportion of the semiconductor material to a temperature such that theband gap energy in the portion is smaller by at least 5% (and morepreferably by at least 10%) than the given band gap energy, the heatingbeing performed so as to generate an inhomogeneous temperaturedistribution within a target volume of the semiconductor. An opticalarrangement directs an optical signal through the target volume toachieve optical amplification.

This aspect of the present invention relates primarily, although notexclusively, to indirect band-gap semiconductor materials, and mostpreferably silicon. It should be noted that the term “silicon” is usedherein in the description and claims, unless otherwise specified, torefer generically to pure silicon crystals and silicon doped withvarious elements as is known in the art. In the case of silicon, thepreferred range of working temperatures for the maximum temperature ofthe heated region is between 200° C. and 1000° C. In certain cases, amaximum temperature in the range of 400° C. to 800° C. may beadvantageous.

The heating arrangement may take a number of different forms. Accordingto a first preferred option, the heating arrangement includes a laserfor directing radiation onto a region of the body. Optionally, a reducedreflectivity compound may be coated onto a region of the semiconductormaterial to increase absorption. A second option for the heatingarrangement is a source of microwave radiation configured for directingmicrowave radiation into a region of the body. A third option employs anarrangement for directing heat from a non-coherent light source, such asan incandescent lamp or from sunlight, onto a region of thesemiconductor material. A fourth option includes an electric circuit forpassing an electric current through a resistive load associated with thebody. In the latter case, the resistive load may be within, depositedonto, or located in proximity to, the semiconductor. Certain exampleswill be discussed below with reference to FIGS. 12A and 12B.

Optionally, the optical signal may be directed into a region of thesemiconductor material coated with a compound having higher reflectivitythan an exposed surface of the semiconductor material so as to causereflection of the optical signal so as to pass through the target volumea plurality of times. This enhances the amplification effect and may beused to construct a laser.

Most preferably, the target volume forms part of an optical waveguideformed in the body of semiconductor material. In this case, the opticalarrangement includes an optical interface for introducing the opticalsignal into the optical waveguide.

Optionally, the non-homogeneous heating mechanism may be implementedwith silicon doped with Gold. In this case, a synergic combination ofthe two effects described herein may advantageously be achieved.

These and other features of the present invention will now be describedfurther with reference to the accompanying drawings.

A. Trap Assisted Stimulated Emission in Silicon A.1. Introduction

As is well known, Shockley-Read-Hall (SRH) and Auger recombination aredominant recombination mechanisms in Silicon. SRH recombination(recombination via deep energy levels in Silicon forbidden gap due tosample deformations, doping, etc.) can enhance radiative recombination.The energy level of impurities or other faults will determine thewavelength of the luminescence. Therefore controlled insertion of dopingtype, traps, or recombination centers can allow the luminescence atspecific wavelength. Moreover, if carriers lifetime in a lower energylevel, where there is recombination of the free electrons and holes, isless then that of an upper energy level (capture center, conductionband, etc.) it would be possible to realize optical amplification orlaser action at specific wavelength.

In this work, we disclose and report the use of Silicon as basicmaterial for lasers and optical amplifiers manufacturing, able tooperate at different wavelengths. Free carriers can be generated byapplying optical pumping, current injection, electrical field (impactionization mechanism), or combination of optical pumping with electricalfield, and by heating of the sample.

Radiative recombination mechanisms for Silicon are still not fullyclarified in some cases. But most of authors agree that luminescence inSilicon is a result of the presence of impurities or faults, that createenergy levels which are situated in the forbidden band. The impuritiescan be acquired due to deformation of the silicon sample, or doping ofthe material. For example, Sauer, et al [10] observeddislocation-related photoluminescence in Silicon. Recombination centerswere obtained by temperature deformation of Silicon samples withdifferent types and concentrations of the doping. Measuredphotoluminescence spectra ranged from 1.1 to 1.7 μm wavelengths. Severalresearchers observed trap assisted luminescence in silicon afterirradiation of the one by electron and proton beams, x and γ rays [11],high power laser beam [12] at that, the luminescence spectra were notequaled to silicon band gap value. Luminescence that is due to a dopinginsertion was observed for gold [13, 14] and silver [15] doped Silicon,as well. In addition, luminescence radiation spectra were well matchedwith familiar data of the energy level positions for these metals inSilicon forbidden band, specifically, 0.35 eV and 0.34 eV above theconduction band, for gold and for silver respectively, Tab.1 [16] showsseveral materials that create energy states in the silicon forbiddengap, which can be involved in stimulated emission process:

TABLE 1 Metal Energy of the state in eV Photon wavelength μm Electronrecombination from Conductive band to Dope energy state with photoncreation Fe 0.4 1.72 Au 0.35 1.61 Ag 0.34 1.59 Cu 0.24 1.41 0.37 1.65 Zn0.31 1.53 0.55 2.1 Co 0.39 1.7 Tl 0.26 1.44 Hg 0.36 1.63 Electronrecombination from Dope energy state to Valence band with photoncreation Ag 0.33 1.57 Ni 0.35 1.61 S 0.37 1.65 0.18 1.32 Mn 0.53 2.1 Fe0.55 2.1 Pt 0.37 1.65 Hg 0.33 1.57

It should be note that, wavelength of luminescence and stimulated lightemission (optical amplification, lasing) can been changed for heatedsamples. No proved method insofar has been given to attain opticalamplification at near-infrared wavelength using doped silicon exceptingrare-earth metals such as Erbium where the properties are attributableto internal energy levels of the metals themselves.

A.2. Theoretical Background

FIG. 1 shows one plausible energy diagram for Gold diffused Silicon,which could account for optical signal amplification. This diagram andmodel is in accordance with observations of Mazzaschi et al [13] whoreported luminescence from gold-doped Silicon at photon energies around0.78 eV and 0.793 eV below the conduction band, very close to the energyof the photons for which we observed amplification. This diagramincludes, in addition to the band structure of Silicon, an recombinationcenter at around 0.32 eV above the Valence Band (E_(Au), ) this level isresult of Gold atoms presence in silicon. We add to the model anadditional energy state of energy E_(D), which is created by phosphorousdoping, close and below the conduction band.

Let's assume that the Silicon sample is illuminated by light pump pulsewith photon energy more or equal than the Silicon band gap. Then,electrons will come over ftom valence to conductive band. During andafter the pump light pulse, electrons are recombined by bothnon-radiative means (Auger and phonon band-to-band recombinationmechanism or via any other traps) and the specific radiative transitionvia the Gold-related recombination center. If a reference signal ofinfrared light with photon energy equal to difference between energyposition of the Gold recombination center and conduction band or donorenergy level (E_(C) or E_(D))-E_(Au)=hv_(Ref)) passes through theilluminated by pump sample, the captured electrons in the recombinationcenter can relax back to the conductive band. As is well-known, freecarriers lifetime is reduced with introducing of Gold in Silicon,therefore we can assume that the free carriers lifetime in recombinationcenter is less then in conductive band. Then, there would be apossibility of stimulated emission in Gold doped Silicon. The presenceof a trap at energy E_(D) of longer lifetime than the conductive band,would favor population inversion and gain. Analogous reasoning would bevalid for Silicon with any other dope impurities that were presented inTab.1. We presented that model as a plausible explanation for theamplification phenomenon we observed, and are aware that other modelsfor explaining it are possible.

A.3. Experimental Details

For testing of our assumption, namely: optical amplification ingold-doped silicon, we used low Phosphorous-doped (N≈10¹³) bond andetchback-Silicon-on-Insulator (BESOI) wafers which were manufactured byShin-Etsu Handotai Co., Ltd. with following geometrical specification:top SOT layer had 5-μm thickness and silicon dioxide buffer layer was0.5-μm.

Gold was thermally diffused in top layer of the SOT wafer at differenttemperature regimes and diffusion times at room atmosphere and pressure.Thickness of the sputtered gold layer was about 1500 A. Diffusion timesfor separated samples ranged from 30 minutes to 7 hours in 30 minutessteps. Diffusion temperature was varied from 550 to 750° C. in 50° C.step for different samples. For some samples we applied fast heating andcooling. Residuary undiffused gold was removed by 3HCl: 1HNO₃ mixture.

After the process the samples were dry oxidized on 0.5-μm silicondioxide layer. The process was useful for smoothing of the surfaceroughness and for elimination of the surface recombination centers.

Next step was waveguide manufacturing. After photolithography process,silicon dioxide was dry etched. Top silicon was wet etched by 72HNO₃:8HF; 20CH₃COOH mixture on 1.5-μm. Both faces of the samples werecleaved. The length of the prepared waveguides was about one centimeter.

FIG. 2 shows the experimental set-up used to demonstrate the invention,and the corresponding apparatus. Briefly stated, the elements shownare: 1) 0.532 mm wavelength pump laser source; 2) mechanical chopper; 3)cylindrical lens; 4) SOI based single-mode waveguide; 5) 1.32 mm or 1.55mm wavelength reference laser sources; 6) single-mode fibers; 7) fastinfrared photoreceiver; 8) oscilloscope; 9) fast visible lightphotoreceiver. Second harmonic of a CW Nd: YAG laser (λ=532 nm) waschosen as pumping light source. The diameter of the output laser beamwas 3-mm, and a cylindrical converging lens with an 18-mm focal lengthwas used for increasing the incident power density of the pumping lightat the sample, down to about a width of 10 μm. Light pump power whichilluminated the 10 μm width waveguide ranged from 10 mW to 1.5 W. Amechanical chopper with 20% duty cycle and frequencies ranging from 1 Hzto 1 KHz was used for modulation of the pumping light. The referenceinfrared light source was tunable laser, ranging in wavelengths between1.527 and 1.576 μm, to provide the optical signal for amplification. Thesignal source was coupled-in into the waveguide by means of asingle-mode fiber. Light coming out from the waveguide was directed intoa fast IR photoreceiver by an optical fiber. The amplitude changing ofthe infrared light was measured by oscilloscope.

A.4. Experimental Results

FIG. 3 shows oscillograms of the infrared reference signal response tothe pump action. Full line trace at the bottom, corresponds to thesituation where the reference signal was switched off while at same timethe modulated pump was present. From the bottom trace one concludes thatno stray light from the pump was detected by the infrared detector atthe recorded sensitivity levels. The dashed line shows infrared signalresponse to the presence of the pump signal. As seen, 6-7-fold signalamplification was evidenced.

FIG. 4 shows transmission of the guided reference infrared signal as afunction of pump power, for both, gold diffused and undiffused waveguidesamples (the last data were taken from Reference [17]). We reportedabout last case in a previous article [17] that dealt with lightattenuation by light in SOI-based waveguides. From comparison betweentwo graphs, it can be seen that undiffused gold changes drastically theoptical properties of the Silicon waveguides inducing opticalamplification instead of induced absorption of the near infrared light.A maximum gain coefficient of 28 dB/cm was obtained at 0.3 W pumpingpower. For pump powers of 0.8 W and higher, attenuation was measuredinstead of amplification, suggesting that free-carrier absorptionmechanisms prevailed. Another possibility for the reduction in gainfactor at high pumping frequencies could be attributed to changes inenergy level spacing generated by high temperature heating of thewaveguides by the incident light [16]. From calculations that werepresented in our previous work [17], free carrier absorption coefficientcan be evaluated to be of about 35 dB/cm (data of the Reference [17]taking into account free carrier lifetime for gold-diffused silicon) ata free carrier concentration (about 10¹⁸ cm⁻³) induced by 0.8 W pumppower in similar waveguide without gold. Thus, we can conclude, thatobserved amplification of the infrared optical signal is due to golddiffusion in the SOI-based waveguides.

In addition it should be noted, that we measured optical amplificationin each gold-doped silicon sample that was manufactured at differentdiffusion process. The observed gain was ranged from 0.5 to 8.5 dBdepending on the specific processing conditions. But dependence of thegain as function of diffusion temperature regime had similar trend withresults of the Reference [14]. The best result, which was presentedhere, was obtained for sample that was manufactured by applying offollowing technology process: fast insertion into furnace at 650° C. andfast withdrawal of the sample after one-hour diffusion time. Weman, etal observed maximal luminescence intensity for 1.54-μm wavelength at thesame diffusion temperature [14].

It should be noted that there may be additional pumping mechanisms inorder to attain gain in an energy scheme where gold or other dopants areincluded. A very advantageous one would be pumping by current injectionin a forwarded-biased p-n junction. Current injection is a common methodto attain amplification in direct band-gap semiconductors.

B. Optical Amplification in Non-Homogeneously Heated SemiconductorMaterials B.1. Introduction

Laser action by homogenously heating of any material, as sole mechanismis not viable since both, Maxwell-Boltzmann and Fermi-Dirac energy levelpopulation laws prevent the attainment of population inversion at anyfinite temperature. On the other hand, laser action by purely thermalpumping is possible provided different temperature regimes are sustainedin the material at different places. Perhaps the best example of purelythermally excited laser system is that of gas-dynamic lasers [18].There, a mixture of gases are heated to temperatures of above 1000° C.and are transported into a colder area by letting it expand through anozzle into a region of lower temperature. Downstream in the expansionprocess regions are found where population inversion is created due todifferent lifetimes of the upper and lower lasing levels. Gas-dynamicCO₂ lasers of this type have produced very high power values. Thisscheme provides an example of a lasing system where the power for itsactivation is delivered purely by heating. No such an effect has beenreported insofar for semiconductor or other solid-state material.

We report here the measurements of gain at near-infrared wavelengths(1.3 μm and 1.5 μm) by non-homogeneously heating of commercial-typesilicon slabs. Due to the uniqueness of our findings, we attempted toattain gain using different means of heating, different irradiationgeometries and different signal sources. We report also the achievementof gain in silicon waveguides of the silicon-on-insulator type.

B.2. Experimental Details and Results

In the reported experiments we tested two-side polished commercial-typesilicon of n and p-types with different thickness and dope levelssamples (suppliers: Motorola, SICO Wafer GmbH).

We took special care in order to minimize spurious effects, which canaffect measurements of optical transmission while irradiating thesamples with a heat or light source. The two prominent effects are:interference Etalon effects in the slab and thermal lensing effects. Tominimize Etalon effects we carried out measurements at a Brewster angleand polarized the signal light in the TM direction. We also preparedsamples with SiO anti-reflection coating at the measured wavelengthrange. Another approach we chose in order to diminish Etalon effects wasto use a low coherent source, namely the spontaneous emission from anErbium-Doped fiber. Mildly thermal focusing effects were observedoccasionally but found to be minor as compared to the measured ones. InCW measurements we used a power meter with area much larger than themeasured beams, and in pulsed experiments we used a lensed fast-responsedetector to gather light from a wide angular range.

As stated, we have encountered optical amplification in severalsituations comprising various heating means and geometric deliveryschemes. We shall in the following describe briefly those schemes andreport on gain attained.

B.2.a. Pulsed laser pumping

We expect laser and optical irradiation at photon-energies greater thanthe forbidden gap to differ from other heat delivery mechanisms, sincehere carriers are not only excited indirectly by thermally inducedprocesses but also by direct generation via photo-absorption. At thewavelengths used for pumping here (λ<1.1 μm), most of the absorption isknown to take place by band-to-band phonon assisted absorption [19].This is a self-augmenting process, since the higher the temperature, thehigher the absorption coefficient [19].

The scheme of edge power delivery is shown in FIG. 5B. A low powercollimated near-infrared signal beam of about 1 mm diameter is directedclose to the border of a silicon slab, perpendicularly to it. At theedge, a pulsed Nd-YAG laser (λ=1.064 μm), of power in the range of from15 mJ to 145 mJ in single a pulse, was mildly focused. This laser beamfunctions as heat source or pump. Time duration of the pump pulse wasabout 50 μsec and the repetition rate was 5 pulses/sec. Amplificationwas measured for different near-infrared light signal sources, namely:1.3-μm and 1.55-μm semiconductor lasers, spontaneous emission fromerbium doped fiber (EDF), (λ˜1.55±0.1 μm) and a ring laser that based onthe EDF with spectral peak at 1.58 μm.

In the experiments we tested commercial two-side polished siliconsamples of n and p-types with different thickness and dope levels. Forelimination of any interference effects, we tested samples with SiOantireflective coating, and for uncoated samples low coherentspontaneous emission from EDF was used as optical signal. In FIG. 6( a)we see oscilloscope traces of the pump and signal pulses following theinteraction. We observe here an 11 percent increase in the transmittedsignal following the interaction. Comparing the maximum transmittedsignal to the value of the signal incident to the sample, we get a gainratio of about 9 percent.

Another feature clear from FIG. 6( a) concerns the timescale of theresponse signal, which is in the millisecond regime. This points outthat the process is temperature-dominated. The maximum value of the gainis attained after termination of the excitation pulse, and the totaltransient duration for this example is of about 30 msec. At a shortertime scale one observes a small transient decrease of signal (about 10percent). This is also a common feature of almost all our experiments.The decrease is attributed to absorption by free-carrier generationdirectly induced by the pump photons [17, 20, 21].

In FIGS. 7( a) and 7(b) we concentrate results of dependence of theoptical gain as function of the excitation power for different samplesand different signal sources. One notices here, that optical gain haslow dependence on the samplers type (thickness and dope type), but thereis a significant difference in the optical gain for samples with andwithout SiO antireflective coating. The marked influence of the coatingon the gain attained is not entirely clear yet. We may attribute theeffect to differences between thermal conductivity between the sampleson interfaces (Si-Air vs. Si—SiO-Air respectively), the heat-transfer atthe relevant temperature being of both radiative and non-radiativetypes. To confirm this hypothesis we conducted crude measurements of thetemperature changes by means of a thermocouple at the backside of thesample that was heated by a continuous green laser. We evidenced thatsilicon with SiO coating is more rapidly heated and to highertemperatures as compared to bare silicon samples. We report also theobservation of gain in uncoated samples tilted at Brewster's angle withrespect of a laser beam with λ=1.55 μm wavelength.

Looking back at FIG. 7( a) one also observes a lower amplification valuefor a 1.3 μm-wavelength signal. Dependence of the optical gain on pumppower for different samples is shown in FIG. 7( b). Here, we can see,that the gain was observed for both p and n-doped silicon samples.Another feature clear from FIG. 7( b) concerns the rising of the gainwith increasing of the sample thickness.

In another configuration, the pump beam was shined into the slab at thesame side of that of the signal beam, forming a variable angle betweenthe two beams (FIG. 5B). This configuration allowed monitoring easilythe amount of overlapping between the two spots. The amount of spatialoverlapping between the signal and pump beam was adjusted by varying thesize of the pump spot and its lateral position. FIGS. 6( b)-6(e) showthe transient response of the transmission following the excitationpulse for different focusing and mutual lateral positions of the signaland laser spots. In all cases the spot of the pump was smaller than thespot of the signal, i.e. the amount of energy delivered to theinteraction region was constant. The transmission however showsdrastically different character, changing from gain to loss while thefocal spot is enlarged. As seen, some of the traces show both, loss andgain, while in some of the cases the gain precedes the loss and in otherthe order is inverted. This provides evidence that the distribution ofthe heating power is clue in the attainment of gain.

In the experiment we tested the one-plane power delivery scheme thatshown in Fig. In the case, pump beam was

We observed optical amplification, as well. The results were notappreciably varied from described here ones.

We point out in addition, that we did not observe optical amplificationwhen Q-switch laser (with pulse duration of about 20 ns) was used asoptical pumping.

B.2.b Quasi-Continuous and Continuous Laser Pumping.

In this case, the scheme of power delivery is similar to that shown inFIG. 5A. At the edge, a CW green laser beam (λ=0.532 μm) of power in therange of 0.5-5 Watt is mildly focused. The pump laser is chopped by aslotted wheel of 2% duty cycle, delivering pulses of trapezoidal shapein time. The duration of these quasi-CW pulses was in the range of0.15-7 msecs and was controlled by the turning speed of the chopper.

In these experiments we observed a linear dependence of gain on the CWpower and gain here reached up to a factor of two. In FIG. 8 we see thedependence of gain on irradiation time. It is observed that the gainincreases with pulse length as expected from temperature-dependenteffect.

Using the same geometrical configuration and the same sources weconducted measurements in a pure CW mode. We encountered gain here tooand its value is shown as a dotted line in FIG. 8.

In addition, in the experiment we tested the one-plane power deliveryscheme that was described in previous section and shown in FIG. 5B. Weobserved optical amplification, as well. The results were notappreciably varied from described here ones.

B.2.c. Optical Gain by Localized Microwave Heating of the SiliconSamples

Our first observations of gain were conducted using optical pumping atphoton energies higher than the Si band-gap, as described in thesections above. Optically-induced stimulated emission in Si is apossibility, but the timing of the pulsed experiments, and theattainment of gain after completion of the pump pulse hinted clearlythat thermal processes are involved. We choose to test this hypothesisby localized heating in a process where no direct photonic excitation ofcharge carriers is involved. A significant gain effect was observed formicrowave spot-irradiation, evidencing that the attainment of gain wasvery likely due of thermal excitation since no optical process wasinvolved here.

B.2.d. Luminescence Measurements

As in usual situations where gain is present, one expects to measurealso exceptional effects of luminescent emission in the form of enhancedspontaneous emission or related effects. This means shutting down thesignal source, and look at possible emission of radiation at therelevant wavelengths induced only by the pump source. First, we pointout that in all the experimental situations described above, no voltagewas observed at the signal detector when the signal source power wasturned off. We took special care in all our experiments to avoid anystray light from the pump to reach the signal detector. In order todetect luminescence induced by the pump source, we built a much moresensitive system: we gathered the light emitted by the sample into amulti-mode fiber by means of a converging lens. The fiber was connectedto a Spectrum Analyzer (Ando model AQ-6315B), with a capability ofmeasuring signals down to −70 dBm. We placed a Si sample of the samekind as the one described in Section B above (CW experiments), and alsoirradiate it with the same CW focused light source (λ=532 nm) from thebackside of the sample. The induced luminescence spectra are shown inFIG. 9 for different power levels of irradiation. Several features areevident:

-   -   The range of wavelengths where luminescence was prominent was        the same as the one where gain was observed.    -   The peak of luminescence for the relevant powers is far-away        from the one predicted by Wien's law for pure blackbody        radiation at the estimated temperatures (for T=1000° K,        λ_(max)=2.9 μm).    -   The shift in the wavelength of maximum luminescence is in        opposite direction to the one predicted by Wien's displacement        law. I.e. here we see peak displacement to longer wavelength        with increasing delivery of power to the sample.    -   The dependence of luminescence intensity on pump power is highly        non-linear.

The spectral shifts with power can be attributed to the shrinking ofband-gap with temperature and has been observed also in transmissionexperiments of uniformly heated semiconductors [19, 22]. We discuss thispoint further in the next section.

B.2.e. Gain Measurements in Optical Waveguides

We are currently performing experiments using waveguides of thesilicon-on-insulator (SOI) type. Measurement setup was liked to opticalscheme that was described in Section A.3. (see FIG. A.2.). Thisconfiguration is especially attractive since light is confined laterallyin two dimensions and the spatial distribution of the optical field isaccurately known. Insofar we have conducted experiments using bothcontinuous and modulated laser sources. We have observed insofartransmission enhancement up to 50 percent in SOI waveguides followingpump irradiation. We point out that gain was explicitly observed inthese cases when the irradiation was displaced from the waveguides byseveral tenths of microns. FIG. 10 shows dependence of an infraredtransmission response at 1.55 μm-wavelength on spatial position of thefocused pump beam respect to waveguide. Here, it possible to see, thatthere is attenuation of the infrared signal, when pump beam directilluminated the waveguide, and about 10% optical amplification wasobserved when focused beam spot was moved from the waveguide place. FIG.11 shows dependence of the signal attenuation and amplification onduration of the pump pulse. Here, we obtained similar dependence of theoptical amplification as function of pulse time in comparison withoptical amplification in bulk silicon (see FIG. 8).

It should be note, that we measured optical amplification in Gold dopedsilicon waveguides when pump spot was removed from waveguide position ontens microns, as well. It seems therefore that in the case of gold-dopedwaveguides both effects were present, namely direct optical excitationand localized heating.

B.2.f. Gain Measurements at Ohmic Contact Heating

FIGS. 12A and 12B show experimental setups for the measurement ofoptical amplification in non-homogeneous heated silicon samples byheating of Ohmic contacts by electrical current. We used Cr as materialfor contacts, which was sputtered on silicon. Thickness of the Cr-layerwas about 300 nm. 60 W DC electrical power supply was connected to theresistor.

In the experiments we measured a few percent of optical amplification.

B.2.f Further Methods for Attaining Gain by Inhomogeneous Heating.

One can think about other methods for attaining gain by the mechanismdescribed here. Among others: heating by a non-coherent light sourcelike a incandescent infrared lamp, halogen lamp, flash lamp orconcentrated sunlight, heating by radio-frequency source, by an electronbeam etc.

Gain Enhancement Schemes

Several gain enhancement schemes which are applied to other situationswhere optical amplification is observed can be applied here too. Worthmentioning is the option of allowing the signal beam to pass more thanonce through the amplifier medium. This can be achieved by the use ofexternal mirrors or incorporating reflecting surfaces by coating thesemiconductor surfaces with a reflecting dielectric or metallic coating.If in such a situation the small-signal gain is equal or larger than thetotal loss, laser action will take place.

B.3. Hypothesis

The following discussion of possible physical mechanisms for theoperation of the present invention are offered merely to facilitateunderstanding. It should be appreciated, however, that the accuracy orotherwise of the proposed mechanisms is inconsequential in view of theobserved result that the invention is operative to produce opticalamplification. The following discussion should therefore not beconstrued in any way to limit the scope of the present invention asdefined in the appended claims.

As stated in the introduction, at this point we have no quantitativemodel to explain our findings. We concentrated our efforts in gatheringa considerable amount of data from diverse situations, all having incommon the fact that power was delivered into Si slabs or waveguides ina non-uniform way. Following all these findings we conjectured anexplanation for these results based on the following main points:

-   -   a. Local bending of energy bands due to non-homogeneous heating        of a semiconductor slab,    -   b. Transfer of charge carriers between the hot spot and        neighboring regions creating areas where the carriers'        distribution is non-thermal (lower temperature), i.e. population        inversion is enabled. Several mechanisms affecting the transport        of carriers are expected in these situations and are discussed        below.

A scheme of the proposed model can be found in FIGS. 13(A) and 13(B).FIG. 13(A) schematically shows energy states deformation in local heatedsemiconductor material. They are shown as discrete states forillustration purposes. Distribution of intrinsic and nonequilibriumcarrier densities for the case are shown in FIG. 13(B).

We discuss now in some detail the mechanisms listed. First, thenarrowing of band gap with temperature is well known [23], and isdescribed properly by the following expression:

$\begin{matrix}{{E_{g}(T)} = {{E_{g}(0)} - \frac{\alpha \cdot T^{2}}{T + \beta}}} & (1)\end{matrix}$

where E_(g)(0)=1.17 eV is the band gap value at 0° K, a=4.73×10⁻⁴, andβ=635 [23]. Translating it to our situation this means that an energygap corresponding to emission/absorption wavelengths of 1.3-μm and1.55-μm, will correspond to heating temperatures of about 800° K and1100° K respectively. A direct measurement of temperature under spotlaser irradiation is not simple. We attempted although, estimation basedon an approximate expression of continuous heating a semi-infinitematerial with a Gaussian laser beam [24];

$\begin{matrix}{T = \frac{P_{opt}d\sqrt{\pi}}{2K}} & (2)\end{matrix}$

Where, P_(opt) is the absorbed pump power per unit area, d is Gaussianradius of the pumping spot, and K is silicon's thermal conductivity.This estimation took us to the expected temperature range.

Our next consideration is about the nature of the measured lightemission. As well known, radiative carriers recombination in Si can berealized by two mechanisms, namely, recombination via carrier trapswithin the forbidden gap or mediated by phonon emission or absorption.Regarding the first possibility, we used commercial standard sampleswith a low concentration of impurities. Dislocations and otherdeformations are also known to enhance emission at the wavelength rangeof interest [10]. For example, Sauer et al. [10] studied and measuredspectral data in the mid-infrared 1.1 μm<λ<1.7 μm on samples wheredislocations where caused systematically by deformations and inclusions.Here, we did not introduce any impurities in silicon samples and did notdeform intentionally the samples before our experiments. We cannot ruleout however the possibility that we introduced temporary strains in thesample during laser illumination or heating, and that these defects hada role in enabling transitions. The second radiative process to beconsidered is band-to-band carrier recombination mediated by absorptionor emission of phonons. Attainment of gain under these processes waspredicted many years ago [5]. Absorption and emission of near IRradiation in clean homogeneously heated samples have been intensivelystudied ([19, 23, 25). The data reported was satisfactorily explained byphonon mediated process and in all cases it was concluded that at highertemperature optical activity was greatly enhanced by gap narrowing, freecarrier generation and phonon generation. At this point we are inclinedto attribute the measured gain and luminescence spectra to last effect,although we are aware that considerable lattice deformations are presentduring the non-homogeneous heating that would facilitate band-to-bandrecombination.

The most essential question in the understanding of the gain effectsobserved is about the mechanism responsible for population inversion andgain. On a first glance the band bending picture (FIG. 13(A)) due tospot heating resembles the one encountered in semiconductor doublehetero-structure lasers, where a region of material with small energygap is sandwiched between two regions with higher gap values. Butcontrarily to that case, here most of the free-carriers are generated atthe central portion of the structure. The dynamics of the free carrierswill determine whether or not population inversion conditions would beattained. We can list here several processes responsible for thedynamics of carriers.

-   -   1. Diffusion of carriers from the center outbound. The carrier        concentration in Si is a very pronounced function of        temperature. Heating from 600° K to 1000° K amounts an increase        in about 2 orders of magnitude in carrier concentration. Thus,        temperature gradients are accompanied by large concentration        gradients, promoting diffusion into the colder regions (FIG.        13(A), direction 1). This process by itself would generate        carrier concentrations (both electrons and holes) at the outer        regions much larger than dictated by Fermi-Dirac statistics in        homogeneous heating.    -   2. Spatial charge generation. The concentration imbalance        created by the diffusion process will generate space charge due        to differences in electron and holes effective masses and        lifetimes. The space-charge generates electrostatic fields        opposing diffusion.    -   3. Level bending forces. The bending of energy levels causes        potential gradients equivalent to static forces that drive the        carriers into the bottom of the potential wells (FIG. 13(A),        direction 2).

Another way to look at the possible attainment of population-inversionis by considering the density of states function in semiconductors.Although this is a fact already implicit in the discussion of carrierdensity, it facilitates the analogy with other laser systems. Thedensity of states interacting with a photon of energy hv is a stronglyrising function of the difference (E_(G)-hv). This is a common featurefor all semiconductors. In the situation we encounter here this fact hasspecial relevance: the central hot spot has both a higher density oflevels and a higher degree of level occupancy as compared to neighboringregions at the same energy. Thus it may be viewed as a populationreservoir in close interaction with displaced radiating levels, andserve a supply of carriers to the neighboring regions.

In conclusion we have presented evidences of the attainment of gain insilicon by non-homogeneous heating of the material. Gain was attainedboth at continuous and pulsed energy delivery, in a variety of heatingconfigurations using different sources and pumps. We have also proposeda mechanism responsible for the attainment of gain observed, based onenergy band bending by temperature inhomogeneity, and dynamics of chargecarriers under this condition. We expect that further research presentlytaking place in our group will consolidate further our understanding ofthese findings and eventually deliver additional lasing schemes.

REFERENCES

-   1 W. Eisfeld, U. Werling, and W. Prettl, Appl. Phys. Lett. 42,    276-278 (1983).-   2 L. Pavesi, Proceedings of Spie 4997, 206-220 (2003).-   3 L. Canham, Nature. 408, 411-412 (2000).-   4 Atanackovic, B. Petar; Marshall, R. Larry, “Devices with optical    gain in silicon”, United States Patent Application 20020048289,    (2002).-   5 N. G. Basov, O. N. Krokhin, and Yu. M. Popov, Sov. Phys. JETP 13,    845 (1961).-   6 T. Trupke, M. A. Green, and P. Wurfel, J. Appl. Phys. 93,    9058-9061 (2003).-   7 Hak-Seung Han, Se-Young Seo, Shin J H, Namkyoo Park., Appl. Phys.    Lett. 81, 3720-3722 (2002).-   8 R. Claps, D. Dimitropoulos, B. Jalali, Electron. Let. 38, 1352    (2002).-   9 I. C. Khoo, R. Normandin, Appl. Phys. Lett. 52, 525 (1988).-   10 R. Sauer, J. Weber, J. Stolz, E. R. Webek, K.-H. Küsters, and H.    Alexander, Appl. Phys. A, 36, 1-13 (1985).-   11 Radiation damage and defects in semiconductors: proceedings of    the International Conference organized by the Institute of physics,    held at the University of reading, 1972.-   12 J. I. Pankove, and J. E. Berkeybeiser, Appl. Phys. Let. 37,    705-706 (1980).-   13 J. Mazzaschi, J. C. Brabant, B. Brousseau, J. Barrau, M.    Brousseau, F. Voillot, and P. Bacuvier, Solid State Communications,    39, 1091-1092 (1981).-   14H. Weman, A. Henry, T. Begum, B. Monemar, O. O. Awadelkarim,    and J. L. Lindström, J. Appl. Phys. 65, 137-145 (1989).-   15 N. Q. Vinh, T. Gregorkiewicz, and K. Thonke, Phys. Rev. B. 65,    033202-1-033202-4 (2001).-   16 S. M. Sze, Physics of Semiconductor Devices, 2nd ed. (New York:    Wiley, 1981).-   17 S. Stepanov, and S. Ruschin, Appl. Phys. Lett. 83, 5151-5153    (2003).-   18 S. A. Losev, Gasdynamic Lasers, (Springer-Verlag Berlin, 1981).-   19 P. J. Timans, J. Appl. Phys., 74, 6353 (1993).-   20 R. A. Soref and J. P. Lorenzo, Technical Digest Series 4, 86    (1989).-   21 R. Normandin, D. C. Houghton, M. Simard-Normandin, and Y. Zhang,    Can. J. Phys. 66, 833 (1988).-   22 V. Alex, S. Finkbeiner, J. Weber, J. Appl. Phys., 79, 6943    (1996).-   23 J. C. Sturm, C. M. Reaves, IEEE Trans. On Electron Devices 39, 81    (1992).-   24 J. F. Ready, Effect of high-power laser radiation, (Academic    Press, 1971).-   25 G. E. Jellison, Jr. Lowndes, D. H. Lowndes, Appl. Phys. Lett. 47,    594 (1982).

1. A method for achieving optical amplification of an optical signalpassing through an indirect-gap semiconductor, the method comprising thesteps of: (a) providing a body of the indirect-gap semiconductor dopedwith at least one element so as to generate at least one added energylevel at a known energy lying within the energy band-gap of thesemiconductor, said added energy level enabling an energy transitionbetween said added energy level and an energy band of the semiconductorcorresponding to generation of a photon of a given wavelength; (b)performing current injection into at least a target region of said bodyof semiconductor; and (c) directing an optical signal of said givenwavelength through said target region.
 2. The method of claim 1, whereinsaid at least one element is chosen from the group comprising: Gold,Silver, Platinum, Iron, Copper, Zinc, Cobalt, Tellurium, Mercury,Nickel, Sulfur and Manganese.
 3. The method of claim 1, wherein said atleast one element is chosen from the group comprising: Gold, Silver andPlatinum.
 4. The method of claim 1, wherein said at least one elementincludes Gold.
 5. The method of claim 1, wherein said indirect-gapsemiconductor is silicon.
 6. An apparatus for achieving opticalamplification of an optical signal passing through an indirect-gapsemiconductor, the apparatus comprising. (a) a body of the indirect-gapsemiconductor doped with at least one element so as to generate at leastone added energy level at a known energy lying within the energyband-gap of the semiconductor, said added energy level enabling anenergy transition between said added energy level and an energy band ofthe semiconductor corresponding to generation of a photon of a givenwavelength; (b) a current injection arrangement deployed for injectingcurrent into at least a target region of said body of semiconductor; and(c) an optical arrangement for directing an optical signal of said givenwavelength through said target region.
 7. The apparatus of claim 6,wherein said at least one element is chosen from the group comprising:Gold, Silver, Platinum, Iron, Copper, Zinc, Cobalt, Tellurium, Mercury,Nickel, Sulfur and Manganese.
 8. The apparatus of claim 6, wherein saidat least one element is chosen from the group comprising: Gold, Silverand Platinum.
 9. The apparatus of claim 6, wherein said at least oneelement includes Gold.
 10. The apparatus of claim 6, wherein saidindirect-gap semiconductor is silicon.